It is well known to fabricate micro-miniature electronic devices in semiconductor material using conventional photolithographic techniques. The resolution capabilities of such standard techniques are limited by interference and diffraction effects that are directly related to the wavelength of the light employed in the photolithographic process. With the advent of the higher density of very large scale integrated circuits, the need arises for sub-micron line widths. However, in practice, the minimum line width which can be accurately replicated by conventional photolithographic printing is 1 to 2 microns. Moreover, to achieve such resolution, intimate contact may be required between a mask and a semiconductor wafer, which, in time, results in physical damage to the wafer and/or mask.
Higher resolution (sub-micron) pattern definition in device fabrication can be achieved with scanning electron beam lithography. However, a fully versatile electron beam exposure system is an expensive and complex installation. Additionally, in such a system, it is necessary that each pattern of each device be exposed in a sequential point-by-point manner in a program-controlled system. Such a procedure is relatively time consuming and expensive.
Accordingly, it has been proposed that a scanning electron beam be used only to generate high-resolution master masks and replication of the mask patterns onto wafers would then be done in some other manner. Soft X-ray exposure systems have been used to replicate the required sub-micron line widths. However, masks to be used in such systems require that the X-ray transparent portions thereof be extremely thin (e.g., 5 to 10 microns) resulting in fragile and dimensionally unstable masks.
U.S. Pat. No. 3,892,973 to Coquin et al. describes a particular mask structure for use in an X-ray lithography system. A thin X-ray transparent film is stretched over and bonded to a support ring and an X-ray absorptive pattern formed thereon. The mask is then positioned proximate a resist-coated wafer and illuminated with X-rays to form a shadow pattern on the resist layer defined by the X-ray absorptive pattern on the thin film. Such a technique has been found to be most effective where only a single mask is required. However, where multiple masking operations at different levels are required, as in the fabrication of a typical integrated circuit, difficulties arise in maintaining registration between mask levels due to dimensional instabilities in each mask.
U.S. Pat. No. 3,742,230 to Spears et al. is directed to a soft X-ray mask having an X-ray opaque support substrate. The support substrate has a plurality of relatively thick webs arranged in a grid fashion to provide support for an X-ray transparent membrane. However, the webs undesirably prevent the exposure of substantial portions of the circuit sites on the semiconductor wafer and such a mask arrangement can be relatively weak in a shear mode when the mask diameter is greater than three inches.
Accordingly, there exists a need for methods and means to expose large diameter wafers to X-ray radiation through a series of masking steps where a number of such masks must register accurately between masking levels while maintaining line width definition in the sub-micron range.